1. Field of the Invention
The present invention relates to a solution film-forming method, in particular to a solution film-forming method for producing a cellulose acylate film used for a liquid crystal display device and a photosensitive material.
2. Description of the Related Art
In a process for producing a cellulose acylate film, a cellulose ester flake is first dispersed in a solvent and the mixture is stirred, preparing a cellulose acylate solution. Next, the cellulose acylate solution is subjected to filtration to remove foreign matters, thereby eliminating the possibility of defects in the film after film-formation. The filtered cellulose acylate solution is then formed to a film by co-casting or the like and the film is dried, thereby producing a cellulose acylate film.
The above-described filtration of the cellulose acylate solution is carried out for the purpose of removing foreign matters in a dope such as undissolved matters and insoluble matters, thereby preventing the occurrence of defects in the formed film. As a filtering material for the filtration, filter paper, filter cloth, sintered metal or the like is used. In any filtering material, pores of the filtering material may be plugged with time, causing a sudden increase of pressure difference in the later half of the filtration. Thus, it is necessary to periodically pass a cleaning solution through the filtering material to clean the filtering material to regenerate it.
Filtering materials having an absolute filtration accuracy of approximately 0.01 mm have been used in the current filtration. The filtration accuracy required is expected to be higher in future. In particular, for the cellulose acylate film for a liquid crystal display of recent years, high quality is required compared with the cellulose acylate film for photography, so that the absolute filtration accuracy needs to be improved.
However, a reduction of the pore size of a filtering material for the purpose of improving the absolute filtration accuracy will soon make the filtering material plugged, extremely reducing the life of filtration. In addition, the process for producing the cellulose acylate film has become faster in recent years, causing a reduction of time until occurrence of filtration plugging. The reduction of the time to the plugging of the filtration material increases cleaning frequency, thereby increasing the load for operators.
Furthermore, when the pore size of a filtering material is reduced, there will be such problems that high filtration pressure is necessary and it takes a long time in filtration, thereby reducing productivity.